Coil-over and Strut Overview

This page documents how I removed the front and rear spring/strut combo, and replaced them with some KYB GR-2's struts and some eBay-special coil-overs. Although this is a fairly straightforward process, there are some problems I ran into while doing it, which I will illustrate.

Springs

As I mentioned, the coil-over spring set came from an eBay distributor (specifically, Dynasty Import, but there are several distributors selling nearly identical kits, with the only apparent difference being spring color). The set was designed for a 92-95 Civic hatchback, but if you do some research, you'll find that these kits are pretty generic; they sell the same kit for many different vehicles.

The kit includes all the pieces you'll need to install and adjust the springs, but no instructions.  Don't bother trying to call the vendor for help, as they already opt out of that by telling you in the auction that the kit comes without instructions, and is "recommended to be installed by a professional."  As this is a generic kit, they include more than you'll be using for the Aspire. 

The spring rate for the set I got is a ridiculously-high non-progressive 500 lbs/in, and all four springs have the same rate. For reference, the stock springs are rated at approximately 110 lbs/in (front) and 90 lbs/in (rear), also non-progressive. This equates to a roughly 400% increase in spring rate, which wouldn't be horribly bad if you had shock absorbers (dampers for those in the UK and other parts) that were stiff enough to control the spring motion. However, the GR-2's aren't nearly stiff enough for this use, and the stock shocks definitely aren't up to the task. More on this later.

Additionally, most vehicles are designed to have a difference in spring rate from front to rear, due to differences in load and jounce/rebound timing. This ensures that not only will the ride be comfortable, but also the tires will maintain contact with the road surface over bumps. Generally, the rear suspension's natural frequency (not the spring rate itself, although that plays a part) is higher (stiffer) than the front, but within limits. If the rear is too much stiffer than the front, the car will feel and handle more like an unloaded truck. As you'll note in the following table, the eBay specials give the car a much stiffer rear suspension (relative to the front) than stock.

Here are some characteristics of these springs, as well as the stock springs, and some coil-overs I had sitting around for a 2nd Generation FWD Eclipse.

The spring rate was calculated using a formula I found in How to Make Your Car Handle by Fred Puhn (© 1985, HP Books - 46):

Where:

This formula only works for non-progressive-rate springs.

By substituting different values for the number of active coils (N), you can determine the new spring rate of cut springs, and thereby tell if they will be stiff enough to prevent bottoming.  You can also use this formula to help select a compatible spring from another vehicle.  N, by the way, doesn't have to be terribly accurate; a measurement to the nearest 1/8 (0.125) of a turn (45º) should suffice.

Note that the measurement for the wire diameter (W) does NOT include the paint. For the measurements in the W row in the above table, I annotated the actual diameter of the wire less the paint in parentheses. Paint thickness will vary somewhat, so you have two options: scrape away the paint on one section of the coil, or just use an estimate.  From my experimentation, I found that paint thickness varies between about 0.010 and 0.020 inches. Unfortunately, the measurements of the wire and the coil diameter have to be very accurate, as even a small difference in either can cause a significant error. For instance, had I included the paint in the wire thickness for the stock front springs, the rate would have gone from 108.6 to 114.3; the same error on the Eclipse front spring results in a rate of 493.9 (vs. 448.5).  If uncertain of paint thickness, it's better to err on the high side (paint thickness of 0.020, for instance).  This prevents you from getting a spring that isn't stiff enough to keep from hitting the bump stops.

To ensure accuracy, a dial, digital, or vernier caliper, or micrometer, with a point- or ball-type anvil should be used to measure the wire diameter (pointed/ball end goes to inside of coil to allow full contact and proper measurement). A disc brake micrometer works well for this purpose, as it allows access to the free coils. The center-to-center coil diameter is a little harder to get correctly, as unless the top and/or bottom coils are mostly flat, you can't take a reading directly from one side to the other at the same point on the coil. This can be measured with a fair degree of accuracy be placing an object on the inside of the coils  that completely contacts the inner surface of the coil (e.g., rolled-up poster board), measuring the outside diameter of that object, and then adding the diameter of the wire (including paint); remember to only add the wire diameter once, not twice.

There are, of course, other ways to measure spring rate.  One way is to measure how far a spring is compressed when a known weight is added to it.  This isn't quite as easy as it sounds, as the surface on which the spring is resting must not deflect with the added weight, and the spring must maintain a perpendicular stance with not only the supporting surface, but also the weight, so that the measurement can be taken correctly.  This gets more difficult with stiffer spring, and also if the mounting surfaces of the top and bottom coils aren't perpendicular to the spring travel, as is the case with most OEM designs.  Another way is to use a compression gauge made specifically for this purpose, but they are usually expensive enough to prohibit their use for all but racing teams and car designers.

Struts

The struts I'm using are KYB GR-2 units, which I purchased from Festiva Motorsport. These are significantly stiffer than stock (approximately twice as stiff), but still give a good ride. They are a good choice for pairing with the FMS progressive-rate lowering springs (which are about 15% stiffer than stock at their stiffest), and should be able to handle cut stock springs as well. Unfortunately, KYB apparently only makes the GR-2's for the Festiva (up to model year 1993). The difference is that the spring perch for the rears is about 1/2" lower on the Festiva, so you may run into tire clearance problems if you have larger-than-stock wheels and tires. I had 205/50-15's on my Aspire at the time, so I had to cut some off the lower spring perch. Unfortunately, modifications of this nature tend to void the warranty on the struts, so you'll have to be 100% committed to this modification once you've done it, and be prepared to buy new replacements when the time comes. 

As good as these struts are, given the extremely high spring rate, they are not a good choice for proper dampening of the coil-overs. In fact, because the struts are so soft (relative to the springs), they will wear out MUCH faster than they would with stock or even FMS lowering springs. Bottom line is, these struts are NOT designed to dampen springs this stiff. If you insist on getting a spring kit similar to this one, be prepared for a bouncy ride, frequent strut replacement, and uneven tire wear.

Also pictured are new front upper strut mounts. They locate the upper part of the struts in the towers while allowing isolation from vibration. Generally speaking, by the time your struts are worn out, these pieces are, too. If yours are all cracked and dry looking, go ahead and spend the money now. The ones shown here are high-durometer (stiffer than stock) replacements from FMS.

Weight Distribution

The weight carried by each corner of the car is of great importance when tuning suspension, as it determines the balance of the car. A 50/50 front-to-rear weight distribution is considered ideal from a performance-driving standpoint.  A vehicle with this kind of weight distribution exhibits neutral handling, meaning that the driver can control tire adhesion and steering angle by throttle and brake inputs, instead of having to turn the wheel more to correct for understeer or oversteer. Although fairly common in sports cars, in the world of normal passenger cars, this type of weight distribution is often just not obtainable without major surgery. However, the weights from diagonally opposite corners of the car can usually be brought close to 50/50 (i.e. left front to right rear vs. right front to left rear). The closer this ratio is to 50/50, the more equal the vehicle's cornering characteristics are between left and right-hand turns.

In stock form, the front-to-rear weight distribution is about 62/38. While not as front-heavy as some FF cars (and actually not terribly more nose-heavy than most FR cars), it's not exactly ideal.  I took my manual-tranny'd car, equipped with AC, on a full tank of gas, to a weigh station. The scale I used had a resolution of 20lbs, and guaranteed accuracy of +/- 20lbs, so the differences in distribution may vary somewhat.  Here are the readings I got:

Worth noting is that the added 160 lbs of my weight was evenly distributed between front and rear.  This means that the car's longitudinal center of gravity is somewhere very near an imaginary line running side to side at the center of the front seats. 

From the above measurements, we can determine the weight carried by each corner:

As you can see, the car is fairly well balanced in stock form, even with the driver. Changing ride height can bring the weight with driver to 50% diagonal distribution. Using stock-type springs and struts, this can be accomplished by using shims at either the upper or lower spring perches. In that case, you'd want to raise the light corner and lower the heavy corner on the same end of the car (e.g., raise the right front and lower the left front). One of the advantages of adjustable coil-overs is that this distribution is much easier to set, as the lower spring perches are adjustable.

For more information on this, I recommend reading Performance Handling by Don Alexander (© 1991, Motorbooks International).

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